The present invention relates to rotary fluid pressure pumps and motors, and more particularly, to an arrangement which substantially improves the sound quality of the noise which occurs during the operation of such pumps and motors. It will be understood by those skilled in the art that the present invention may be utilized with various types of pumping and motoring elements. For example, the present invention may be utilized with radial ball or radial piston pumps and motors. However, the invention is especially advantageous when used in an axial piston pump, and will be described in connection therewith.
In a typical axial piston pump, there is a rotating cylinder barrel, which includes a plurality (typically, an odd number) of reciprocating pistons. The pistons engage a cam or swash plate, the position of which may be varied to adjust the displacement of the Dump The end of the cylinder barrel opposite 15 the swash plate is seated against a valve plate which defines a fluid inlet and a fluid outlet. The inlet and outlet are connected, respectively, to the pump inlet port and the pump outlet port defined by the housing.
In a conventional axial piston pump, the cylinders are equally spaced, circumferentially, and at the end of each cylinder, the cylinder barrel defines a cylinder port or kidney port, which provides fluid communication between its respective cylinder and the fluid inlet and fluid outlet in the adjacent valve plate. In a typical, prior art axial piston pump, each of the cylinder barrel kidney ports is the same size, in both the radial and circumferential dimension, with circumferential dimension of each kidney port being substantially equal to the diameter of the cylinder. See, for example, U.S. Pat. No. 3,274,897, which is incorporated herein by reference.
Axial piston pumps of the typo described above have been in widespread commercial use for many years, and have been quite successful commercially. Furthermore, their functional performance has been considered generally quite acceptable. However, with increasing concern regarding environmental issues, such as noise, there has been an increasing effort to reduce the noise produced by vehicle components, such as pumps and motors.
More specifically, axial piston pumps and motors produce a characteristic high frequency noise which is generally considered quite objectionable, and which results partly from the sequential compression and decompression of hydraulic fluid in the piston chamber. One result of such compression and decompression of fluid is vibration of the cam plate (swash plate). It is generally recognized that the compression and decompression phenomenon, and the resulting cam plate vibration, can be controlled by varying valve plate timing, i.e., the initiation of communication between the kidney port and the fluid inlet or fluid outlet in the valve plate. However, varying valve plate timing during operation of an axial piston pump requires rotating the valve plate; while it is under heavy axial loading from the cylinder barrel, which is not very practical.